This invention relates to an internally cooled turbine blade for use in high temperature gas turbines. The turbine blade unit of this invention is particularly related to those designs which admit both liquid and air into the turbine blade for mixing and subsequent discharge into a combustion chamber. The construction and operation of the blade unit of this invention is, in general, similar to the blade design described in my patent entitled, Gas Turbine Engine Operating Process, U.S. Pat. No. 4,845,951, issued Jul. 11, 1989, and in particular to my application entitled, Gas Turbine Engine, Ser. No. 348,674, filed May 8, 1989.
The turbine blade unit of this application is particularly designed for use in turbine engines having an annular combustion chamber arranged around the periphery of the ends of a turbine blade assembly. In this arrangement a liquid and air mixture that is utilized to cool the blades can be discharged directly from the end of the blades into the annular combustion chamber. Preferably, the combusted and heated gases in the combustion chamber are then directed at the blades that have been cooled to drive the turbine.
The turbine blade unit of this invention is of the general type as disclosed in the earlier prior art patent of Triebbnigg, et al, entitled, "Method and Apparatus for Internally Cooling Gas Turbine Blades with Air, Fuel, and Water," U.S. Pat. No. 2,647,368, issued Aug. 4, 1953. In the Triebbnigg reference, there is described a turbine blade having passages where air and water or fuel are mixed in the root of the blade and passed linearly through the body of the blade for discharge at the end of the blade to downstream gas flow. In a preferred example where fuel is used as the liquid coolant, the discharged air-fuel mixture is subsequently combusted in an after-burner assembly.
In the preferred environment of a turbine engine with an annular combustion chamber as described in my prior patent, the liquid-air mixture, after cooling the turbine blades is discharged as a vapor-air mixture directly into the combustion chamber. When the blades are incorporated in a blade assembly on a rotor, the 15 discharge is at a high velocity that enhances the pressurization and turbulent mixing of gases in the combustion chamber for a complete and thorough combustion. It is contemplated that additional air and fuel are supplied under pressure in a controlled manner directly to the combustion chamber in order to optimize the combustion for the particular operating conditions of the turbine engine, in part depending on the type of liquid used in the air-liquid coolant mixture.
Because an optimized, substantially stoichiometric combustion of fuel in a turbine combustion chamber may generate heat that exceeds the structural limitations of uncooled turbine blades, various means have been devised to optimize cooling and therefore maximize the efficiency of operation of the turbine engine. Coolants, such as air, fuel, water or combinations of these fluids have been utilized successfully to cool turbine blades and raise the operating temperature of the combustion chamber. In general, stoichiometric combustion of typical turbine fuel will produce a temperature of approximately 4000.degree. F. To maintain structural integrity of turbine blade materials, high temperature metal alloys such as Inconel have been developed which operate at 1800.degree. F. and above. To optimize the thermal environment in which such blades can be operated, improved designs which capitalize on the vaporization of liquids in an air stream are desired. Since a primary use for turbine engines is in aircraft, it is economic to utilize the fuel as the desired coolant prior to combustion. In this manner the heat required to vaporize the fuel and bring it to a temperature that will not dampen efficient combustion is extracted during the process of cooling the turbine blades. This regenerative process greatly enhances the overall efficiency of the engine.
In situations where weight is not a problem, such as cogeneration, the coolant can be a non-combustible such as water, and the engine operated in a combined Brayton cycle and Rankine cycle. In the preferred embodiments, a combined liquid and air mixture is desired for the coolant because of the recuperative effect in recovering the energy necessarily required to bring the liquid-air mixture to a gaseous state for use as a motive medium to drive the turbine blade assembly that has been cooled.
In the embodiments defined in my prior application, referenced above, a liquid coolant was delivered through a center passage in the blade unit and dispersed through diagonal side passages to outer channels between the core of the blade and an outer skin that is shaped to give the blade its dynamic airfoil configuration. It has been discovered, that the zone of mixing between the blade core and the outer skin was not optimized because of the essentially laminar flow of air and fuel mixture moving rapidly through the channels. Because of the very short dwell time of the fuel and air in the channels, cooling of the turbine blade skin, the component that forms the airfoil that is exposed directly to the heated combustion gases, was impeded by the laminar flow.
The particular embodiments of my previous application, Ser. No. 348,674, utilized a central cavity that was primarily a fuel cavity wherein the liquid fuel in the cavity was forced by compressed air through diagonal passages to channels between the blade core and blade skin. The fuel dispersed into the channels was transported through the linear channels between the blade core and blade skin for discharge at the end of the blade core. The initial deployment of air into the linear blade channels reduced the effectiveness of the incoming fuel from the staged diagonal passages, apparently because of laminar conditions in the flow.
In the embodiments of this invention, a dual liquid-air path is provided. By a first path, compressed air is supplied to a central air cavity and fuel is sprayed into the cavity, directed at the wall of the cavity. Simultaneously, by a second path, fuel is sprayed into the entry of cooling channels between the blade core and the airfoil skin. Cross passages between the central cavity and the channels provide for a colliding mixture of partially vaporized liquid and air in the channels disrupting 10 laminar flow. The turbulent flow of the liquid-air mixture through the channels cools the skin and outer surface of the core before discharge into the combustion chamber. It is this turbulent cross flow that is most effective in rapidly vaporizing the liquid fuel film formed on the walls of the channels.